A tuner is a frequency translation device which translates incoming RF signals from one frequency to a typically lower frequency. The information content of the received RF signal is normally modulated is some fashion on a carrier wave and the tuner serves to demodulate the RF carrier in order to extract the original data stream. Tuners are used in practically every wireless transmitting/receiving device, particularly in the consumer product market.
For example, TV tuners are found in set top boxes, televisions, VCRs, as well as cable modems and other wide bandwidth access devices. These tuners take in a particular high frequency signal, translate and filter the signal, to a lower frequency signal, which is typically a fixed frequency for a particular application. The output of the tuner at the second frequency (called an intermediate frequency) is then available for further processing.
For most consumer applications, the price of the tuner is one of the driving motivations. The price drives the acceptance for use in the consumer market and tuner manufacturers must balance the price/performance model. With price in mind, there are many different topologies to achieve the functionality required by the tuner. Certain shortcuts or compromises must be made in terms of the hardware realization of the tuner in order to achieve competitive prices/performances.
In the field of cable modems, there are two basic broad categories of tuners in current use. One of those is what's commonly referred to as a double conversion tuner, which has two frequency translations. Between these two frequency translations that is a fixed, (typically higher) frequency where the filtering is achieved. The input of these devices is typically very wide band.
The other approach is what's commonly called a single conversion tuner. Single conversion tuners have tracking filters at the input that track the frequency of the signal, thereby reducing the total signal power on the input of the tuner. Single conversion tuners have one frequency translation direct from the incoming RF to a fixed frequency where filtering occurs. Traditionally, single conversion tuners are used in terrestrial devices, such as televisions, VCR receivers, etc. Through economies of scale due to the large number of these tuners, they tend to be the lower cost alternatives.
The single conversion tuner, however, because it does have tracking filters, requires a different architecture than does the double conversion tuner. One of these architectures is that the entire frequency band, which is very wide covering roughly from 50 to almost 900 megahertz, must be split into several bands in order to achieve the necessary tracking filter functionality. Since tuners are used as a “front end” to other devices it is important to calibrate the gain of the tuner to subsequent circuitry for proper performance. Thus, it is necessary to know the “typical” gain for a tuner to achieve proper overall system operation.
One drawback of single conversion tuners is that the gain of the tuner, that is the gain from the input signal to the output signal of the tuner as a whole, varies significantly over frequency, temperature and other aspects. The gain varies not only within the tuner across frequencies, (mainly because different circuits handle different bands) but also the characteristic and absolute gain of a particular frequency varies from tuner to tuner. Current tuners are typically set to provide a gain variation across all receive frequencies of a maximum of 8 dbs. As discussed, this presents a problem in that the gain is not repeatable from tuner to tuner nor within bands of the same tuner. Therefore, to extrapolate what a given gain will be at a given frequency of the tuner is difficult without actually measuring the gain of that device. Therefore, when tuners are selected at random and when a given frequency is selected it is difficult to know what the gain of that tuner will be, except that it will lie within the specified 8 db of gain range.
Due to the switching between bands of a tuner, there is discontinuity in gain between the higher end of one band and the lower end of the next band. One method of working around this problem is to have the demodulator which follows the tuner device extract the digital information from the frequency translated signal by a closed loop system. This means that although the absolute gain of the tuner over frequency may vary up to 8 db, the closed loop function of the demodulator will control the system such that as far as the demodulator input sees, the power is constant. These applications are acceptable for approximately 95% of current users.
The new DOCSIS Specification (DOCSIS1.1) has a number of modifications, many of which deal with software modifications that are irrelevant to the tuner functionality. However, there is one addition or modification to this specification which does have significant impact on the tuner, and that is a requirement allowing cable companies or others, to communicate with the modem end user device and poll this device to determine the incoming signal level to that modem. This power measurement capability will allow the cable company to troubleshoot their network since they could, in theory, find the power level at each end user on their cable plant, and in such a fashion, determine information regarding the quality of their signal, as well as any problems or interruptions in that signal.
Specifications for this ability to measure the power are in terms of an absolute gain variation. In other words, it is desired to determine the input power level within the variation of, for example, plus or minus 2 db. In addition, it is desired to be able to step change the power levels to within a certain specified resolution. For example, for a step increase of 1 db of power, the end user device should be able to report a modification in gain of 1 db plus or minus ½ db. These requirements are contrary to the gain variation of current tuners, since, as discussed above, tuners typically have a gain variation of 8 db.
In order to achieve accurate downstream power measurement the cable operator must perform (or have performed by the modem vendor) a very detailed, multi-point power calibration for each tuner/demodulator combination. Because of the fact that the measurement of power from the operator can be performed at any time and at any input power frequency, the closed loop gain variation controlled by the demodulator and which controls the gain of the tuner may be at any arbitrary setting. This requires that the end unit cable modem be able to report the input power level within the specified accuracy regardless of the input power, as long as that is within the specified range.
Due to the gain variations across frequency, and also the non-linear behavior of the gain control AGC function, it is necessary to derive a matrix of data gain versus frequency and gain versus AGC control voltage, and store this in non-volatile memory existing within the end user's device. Currently, the end user measures the input power through reading of a register within the demodulator that is mounted on their cable modem. One drawback of this approach is that the reading of this downstream power measurement is very slow—the order of three minutes per device. Each device must be separately calibrated and the data stored therein. Once the data is stored, a simple table lookup procedure based upon frequency can be used to provide correction for power measurement determinations.
In typical RF receivers, there are two variable gain stages. The first is the RF amplifier, sometimes referred to as a Low Noise Amplifier, and the second is the IF (intermediate frequency) amplifier. To maximize the signal-to-noise-ratio (SNR), it is desirable for the RF amplifier to operate at maximum gain. However, if the input signal becomes large, it may overload the circuits downstream of the RF amp. Thus, there is some input signal level where it becomes necessary to roll off the gain of the RF amp. This point is called the TOP. In general, engineers defining receiver AGC characteristics call for the RF amp to operate at full gain for weaker signals and account for increasing signal strength by first reducing the gain of the IF amp. Once the IF amp is operating at a minimum gain setting, then the gain of the RF amp is reduced. Because of the variation of gain versus frequency and AGC setting in the RF amp one TOP setting for all channels does not result in the best overall performance.
One solution is to reduce the necessity of performing multi-point calibration by reducing the amount of allowed gain variation across the frequency range in the tuner itself. However, due to the fact that single conversion tuners are typically manually adjusted through the alignment of coils within the tuner and the fact that multiple operators align tuners at a given time, and given the number of degrees of freedom available within a tuner in order to achieve correct tracking and alignment, it is not feasible within a high production environment to achieve the necessary tolerance.